The present invention relates to a comparator device applicable for switching control for a plurality of paths in a multi-path transmission system receiving an identical modulation signal through a plurality of receiving sections, such as a diversity reception system.
In general, a comparator device to be applied for such type of a diversity reception system includes first and second comparator means having identical hysteresis characteristics. With these comparator means, voltages corresponding to receiving conditions at two transmission paths are compared to control transmission means for transmitting a transmitting signal based on respective comparator outputs. Control is typically performed to switch one of the comparator means from a conducting state to a non-conducting state and the other comparator means from a non-conducting state to a conducting state. Upon input, both of the transmission means for the transmitting signal temporarily become conductive for avoiding drop out of the transmitting signal or click noise due to delay in response at the transmission means.
Here, a diversity receiver having typical two receiving sections, for which the conventional comparator device is applied, is schematically illustrated in FIG. 1.
In the diversity receiver illustrated in FIG. 1, the reference numerals 1 and 2 denote first and second antennas corresponding to respective receiving channels for receiving a common modulation signal, 10 and 20 denote first and second receiver means respectively corresponding to the receiving channels. Through respective receiving channels, first and second demodulation signal outputs of the received signals, and first and second receiving condition signals V.sub.1 and V.sub.2 corresponding to the receiving conditions of respective receiving channels are obtained. The first and second receiving condition signals V.sub.1 and V.sub.2 have characteristics to be greater for better receiving condition.
The reference numerals 11 and 21 denote first and second variable transmission means for transmitting the first and second demodulated signals from the first and second receiving means 10 and 20 to a common output terminal 14.
The reference numerals 12 and 22 denote first and second comparator means which have identical hysteresis characteristics. Respective pairs of inputs of the first and second comparator means 12 and 22 have mutually different polarities and the inputs having the different polarities are connected to each other. The first and second receiving condition signals V.sub.1 and V.sub.2 are supplied from the first and second receiver means 10 and 20 to respective inputs of the first and second comparator means 12 and 22. The first and second comparator means 12 and 22 compare the inputs and respectively output first and second comparator output condition signals S.sub.1 and S.sub.2. The outputs of the first and second comparator means 12 and 22 are respectively supplied to corresponding ones of the first and second variable transmission means 11 and 21 so that the variable transmission means is controlled into a conducting state in response to HIGH level (hereinafter simply referred to as "H") of the corresponding ones of the first and second comparator output condition signals S.sub.1 and S.sub.2, and into a non-conducting state in response to LOW level (hereinafter simply referred to as "L") of the corresponding ones of the first and second comparating output condition signals S.sub.1 and S.sub.2. Therefore, one of the first demodulated signal from the first receiver means 10 and the second demodulated signal from the second receiver means 20 is selectively transmitted to the output terminal 14.
Subsequently, the operation of the conventional diversity receiver having the construction as set forth above will be discussed with reference to FIG. 2.
FIG. 2 illustrates the relationship between the first and second receiving condition signals V.sub.1 and V.sub.2 as the inputs for the first and second comparator means 12 and 22 and the first and second comparator outputs condition signals S.sub.1 and S.sub.2 as the outputs thereof.
Here, (a) and (b) of FIG. 2 shows the relationship between the threshold values of the first and second comparing means 12 and 22 having identical hysteresis characteristics and the comparator outputs condition signals S.sub.1 and S.sub.2, with respect to the first receiving condition signal V.sub.1. For these first and second comparator means 12 and 22, the same threshold values are provided. The threshold values for each of the comparator means 12 and 22 are determined such that, taking the non-inverting input thereof as a reference value, an average of two threshold values of each comparator means, i.e. one half of the sum of the threshold values, is greater than the reference value, and the reference value is present between two threshold values. Therefore, when the inverting inputs have a value equal to the reference value, the comparator output condition signal of the comparator means can be either H or L.
In case that the difference polarities of inputs of the first and second comparator means 12 and 22 are connected to the first and second receiver means 10 and 20 and taking the first receiving condition signal V.sub.1 as a reference, the relationship of the levels of the threshold values in the second comparing means 22 is reversed so that the average value of two threshold values becomes smaller than the reference value as can be seen in (b) of FIG. 2. Therefore, when such connection is established, the threshold values of respective ones of the first and second comparing means 12 and 22 should have mutually different values to perform the operations as illustrated in (a) and (b) of FIG. 2.
Namely, assuming V.sub.2 &lt;&lt;V.sub.1, S.sub.1 =H and S.sub.2 =L are established so that the demodulated signal of the receiver means 10 is supplied to the output terminal 14.
Here, when the level of the second receiving condition signal V.sub.2 rises to reach the threshold value across which output of the comparator means 22 is switched from L to H, S.sub.1 =H and S.sub.2 =H are established so that the first demodulated signal of the first receiver means 10 and the second demodulated signal of the second receiver means 20 are supplied to the output terminal 14 in parallel. At this time, since each of the demodulated signals is derived through demodulation of the same modulation signals and thus should have the same amplitude, such parallel supply condition of the demodulated signals will never cause variation of the amplitude. When the level of the second receiving condition signal V.sub.2 further rises to reach the threshold value, across which the first comparator output condition signal of the first comparator means 12 is switched from H to L, S.sub.1 =L and S.sub.2 =H are established so that the second demodulated signal of the second receiver means 20 is supplied to the output terminal 14 and the first demodulated signal of the first receiver means 10 is disconnected from the output terminal 14 for completing the switching operation.
In FIG. 2, (c) and (d) show all possible transitions of the first and second comparator output condition signals S.sub.1 and S.sub.2 relative to variation of the second receiving condition signal V.sub.2 taking the first receiving condition signal V.sub.1 as a reference. As can be seen therefrom, at the transition according to variation of the first and second receiving condition signals V.sub.1 and V.sub.2 for switching supply of the demodulated signals from that of the first receiving means 10 to that of the second receiving means, and vice versa, both of the first and second demodulated signals are supplied to the output terminal in parallel, temporarily. Since the hysteresis of respective ones of the comparator means 12 and 22 will not change even when the amplitude of the first receiving condition signal V.sub.1, taken as a reference, changes, the condition of the operation is not dependent on the first receiving condition signal V.sub.1.
As set forth above, even in the conventional comparator device, as applied to the diversity receiver, both of the demodulated signals are temporarily transmitted at the transition for switching demodulated outputs of two receiver means for avoiding occurrence of signal drop out in click noise due to delay of operation in the transmission means. Also, since it utilizes the hysteresis characteristics, the transmission state, in which both of the demodulated outputs are transmitted, is past at a speed proportional to the speed of variation even at high variation speed of the receiving condition signals and thus can follow even for substantial variation of the switching frequency.
However, in the conventional comparator device constructed as set forth above, as can be seen in (d) of FIG. 2, a problem is encountered in that the condition, in which none of the demodulated signals of the first and second receiver means 10 and 20 is transmitted, i.e., the condition where both of the comparator output condition signals S.sub.1 and S.sub.2 are L, resides in the proximity of the important portion where the receiving condition signals V.sub.1 and V.sub.2 are close to each other. Though such condition may not be introduced during normal operation, there is a possibility of introduction as affected by noise and so forth. This makes it difficult to guarantee reception which is the fundamental task of the diversity reception system.